X-ray imaging is widely deployed in various fields including medical imaging, security screening and industrial non-destructive testing. The following discussion is focused on security screening although the same approach applies to all other X-ray inspection fields.
Radiographic images are produced by the detection of radiation that is transmitted through or scattered from the object being inspected. The density, atomic number and the total amount of material that is present determine the extent of attenuation of the radiation and, therefore, the nature and type of radiographic image produced. In addition to determining the average absorption of the X-ray or gamma-ray photons as they travel along the various X-ray paths, it is possible to derive information about the characteristics of the material. The intensity of scattered X-rays is related to the atomic number (Z) of the material scattering the X-rays. In general, for atomic numbers less than 25, the intensity of X-ray backscatter, or X-ray reflectance, decreases with increasing atomic number. On the other hand, materials with high atomic number (Z>70) are characterized by high attenuation of the low and high end of the X-ray spectrum. Therefore, X-ray images are primarily modulated by variations in the atomic number of items of various materials inside an object (such as within cargo).
As a result of the image modulation according to atomic numbers of various materials, it is common for X-ray imaging systems to produce images with dark areas. Although these dark areas might indicate presence of threat materials, they yield little information about the exact nature of threat. Also, radiographs produced by conventional X-ray systems are often difficult to interpret because objects are superimposed and may confound the image. Therefore, a trained operator must study and interpret each image to render an opinion on whether or not a target of interest, such as a threat, is present. Operator fatigue and distraction can compromise detection performance when a large number of such radiographs are to be interpreted, such as at high traffic transit points and ports. Even with automated systems, it becomes difficult to comply with the implied requirement to keep the number of false alarms low, when the system is operated at high throughputs.
One method of obtaining more useful information and clarity from X-ray imaging is using dual energy systems to measure the effective atomic numbers of materials in containers or luggage. Here, the X-ray beam is separated into two broad categories: low energy and high energy. Often this is achieved by passing the X-ray beam through a first thin X-ray detector that responds preferentially to low-energy X-radiation. This filtered beam is then passed to a second detector which responds to the filtered beam which is weighted towards the higher energy part of the spectrum. Effective atomic number is then determined by taking the difference between the high energy and low energy signals. This is particularly effective for X-ray energy beams in the range of 60 kV to 450 kV where the rapid change in the linear attenuation coefficient of the object under inspection yields good contrast between the low and high energy spectral regions.
An alternative technique uses one X-ray detector per imaging pixel, however, varying the beam quality projected from the source as a function of time. Typically, this technique is used in high energy inspection systems (e.g. 4 MV and 6 MV beams). Since the effective atomic number is now generated from the high energy part of the spectrum, it is able to provide information over much thicker regions of the object than is possible with the dual energy, stacked detector, approach.
A further approach recognizes that the X-ray beam comprises multiple individual X-ray photons which interact with the detector material at different, randomly dispersed, moments in time. By detecting and processing each interacting X-ray photon separately, it is possible to build an energy spectrum of all the interacting X-rays, where this spectrum contains many discrete energy bins, typically in a range of 2 to at least 4 bins. It is then possible to analyze the shape of this spectrum to determine effective atomic number of the objects under inspection. Commonly known detection systems use scintillator and semiconductor materials to generate such radiation spectra.
Such spectral sensing systems generally are very expensive to procure since they combine several individual components including, but not limited to, detector material, analog signal processing circuits, analog to digital converters, digital signal processing circuits and digital data acquisition circuits. These circuits tend to be of high bandwidth in order to achieve the designed counting rates, typically up to 100 million counts per second per square millimeter, and thus are power hungry and space consuming. The systems also tend to have poor manufacturing yield and are often sensitive to changes in ambient temperature, particularly due to leakage current variation with temperature in the detecting materials.
Accordingly, there is a need for improved multiple energy detectors that are less power intensive and space consuming. There is further a need for detectors that not only reduce the cost and improve manufacturing yield, but are also able to mitigate against temperature variations.